1. Field of the Invention
The present invention relates to a polishing apparatus and method for polishing a surface of a workpiece such as a semiconductor wafer, and more particularly to a polishing apparatus and method having a function of polishing a surface of a film formed on a substrate to a flat mirror finish and a function of polishing unnecessary metal film or the like, particularly copper (Cu) film, deposited on an outer peripheral portion of the substrate to remove such unnecessary metal film. The present invention also relates to an outer periphery polishing apparatus for polishing an outer peripheral portion of a substrate to remove unnecessary metal film or the like deposited on the outer peripheral portion of the substrate.
2. Description of the Related Art
Generally, aluminum or aluminum alloys have been used as a material for forming interconnects (interconnections or wiring) for a semiconductor substrate. However, in recent years, there has been a growing tendency to replace aluminum or aluminum alloys with copper having a low electric resistivity and a high electromigration resistance. Copper interconnects of this kind are generally formed by filling copper into minute trenches or via holes preformed in the surface of the substrate. As a means for filling copper into the minute trenches or via holes, there are various methods including CVD, sputtering, and plating, and in every method, copper is deposited over a substantially entire surface of the substrate, and then unnecessary copper is removed by a chemical mechanical polishing (CMP).
FIGS. 13A through 13C show successive steps of manufacturing copper interconnects in a substrate such as a semiconductor wafer. As shown in FIG. 13A, an oxide film 2 of SiO2 is deposited on a conductive layer 1a on a semiconductor substrate 1 on which semiconductor devices are formed. Then, a via hole 3 and a trench 4 for a predetermined interconnect pattern are formed in the oxide film 2 by lithography and etching. Thereafter, a barrier layer 5 of TiN or TaN or the like is formed, and then a seed layer 7 to supply electric current for electroplating is formed on the barrier layer 5 by film deposition.
Then, as shown in FIG. 13B, the entire surface of the semiconductor substrate 1 is coated with copper by electroplating to deposit a copper layer 6 on the entire surface, thus filling the via hole 3 and the trench 4 with copper. Thereafter, the copper layer 6 on the barrier layer 5 and the barrier layer 5 are removed by chemical mechanical polishing (CMP), thus making the copper layer 6 in the via hole 3 and the trench 4 lie flush with the oxide film 2. In this manner, an interconnect composed of the copper layer 6 is produced as shown in FIG. 13C.
In this case, the barrier layer 5 is formed so as to cover substantially the entire surface of the oxide film 2, and the seed layer 7 is also formed so as to cover substantially the entire surface of the barrier layer 5. Thus, in some cases, as shown in FIG. 14, a sputtering film of copper which is the seed layer 7 resides in a bevel (outer peripheral portion) of the substrate W, or copper is deposited on an edge (outer peripheral portion) inwardly of the bevel and remains unpolished (not shown in FIG. 14).
Copper can easily be diffused into an oxide film in a semiconductor fabrication process such as annealing, thus deteriorating the electric insulation of the oxide film and impairing the adhesiveness of the oxide film with a film to be subsequently deposited to possibly cause separation of the deposited film. It is therefore necessary to remove the remaining unnecessary copper completely from the substrate before at least film deposition. Furthermore, copper deposited on the outer peripheral portion of the substrate other than the circuit formation area is not only unnecessary, but may cause cross contamination in subsequent processes of delivering, storing and processing the substrate. For these reasons, it is necessary that the remaining deposited copper on the peripheral portion of the substrate be completely removed immediately after the copper film deposition process or the CMP process.
Here, the outer peripheral portion of the substrate is defined as an area including an edge and a bevel of the substrate, or either the edge or the bevel. The edge of the substrate means areas of the front and back surfaces within about 5 mm from the outer peripheral end of the substrate, and the bevel of the substrate means an area of the outer peripheral end surface and a curved portion in a cross section within 0.5 mm from the outer peripheral end of the substrate.
There has been generally known an edge polisher for polishing a bare silicon wafer, but there has heretofore been no polishing apparatus for polishing only an outer peripheral portion of a substrate having a film formed thereon.
This kind of edge polisher for polishing a bare silicon wafer generally comprises a substrate holder and a polishing member. In the embodiment shown in FIG. 15, the substrate holder 304 is composed of a robot hand 302 which extends vertically and has a holding portion 300 at a lower end thereof, and is movable in vertical and horizontal directions. The substrate W such as a silicon wafer is held by the holding portion 300 of the robot hand 302 under vacuum in such a state that the substrate W is inclined with respect to the horizontal plane and the edge of the substrate W is externally exposed. The polishing member 312 comprises a cylindrical support member 306 and a polishing cloth 308, and has a polishing surface 310 which is formed by a surface of the polishing cloth 308 affixed on a side surface of the cylindrical support member 306. With this arrangement, while the edge of the substrate W is brought into contact with the polishing surface 310, at least one of the substrate holder 304 and the support member 306 is rotated, and the substrate W is simultaneously moved up and down along the polishing surface 310 by the substrate holder 304, thereby polishing the edge of the substrate W.
When the entire surface of the edge of the substrate W is polished by this kind of edge polisher, one side of the edge of the substrate W is polished, and then the substrate W is reversed and held again by the substrate holding member 304. Thereafter, the other side of the edge of the substrate W is polished.
Therefore, if the outer peripheral portion of the substrate having a film formed thereon is polished by the edge polisher for polishing a bare silicon wafer, this polishing work is required to be performed separately at a location different from the polishing work of the surface of the substrate. Thus, this polishing work is considerably troublesome. Further, in this kind of edge polisher, the outer peripheral portion of the substrate (workpiece) is polished while the outer peripheral portion of the substrate is brought into point contact with the polishing surface at a point along a circumferential direction of the substrate, and hence the polishing rate thereof is generally low and it takes much time to complete polishing.